I want to join two accounts into one, so that the balance of the new account became the sum of their balances (and the set of transactions would become the union of their sets of transactions). It seems now there is no way to do this. Please add this operation.
Open the ledger of the account you want to merge into the other. Press Ctrl+A to select all transactions. Right click the mouse and select 'Move transaction to ...' and select the second account in the drop down box that opens up. Remove the first account and you're set.